The art is filled by various types of devices for measuring the level of a liquid contained in a tank or similar reservoir. Some of these devices provide a continuous or instantaneous indication of the liquid level so that an operator or monitoring system is provided with liquid level information at all times. One type of known liquid level sensor merely provides an indication of when the liquid level falls or rises to a pre-elected level indicating, for example, that the liquid level is either too high or too low. This latter mentioned level sensor is often used to control the delivery of liquid from a source into the tank in order to replenish the tank. In such applications, two discrete levels sensor are often employed, one being used to shut off the replenishment supply when the level reaches an upper level, and the other being used to energize a replenishment pump when the liquid level falls to some lower threshold value.
Discrete liquid level sensors are known which employ mechanical float systems used to actuate electric pumps, but these systems are complex in design and often require submersion of many of the moveable mechanical components of the sensor. In order to avoid this problem, others have resorted to non-contact type sensors which operate on the principle of measuring changes in capacitance or inductance, however these level sensors are undesirable in that they are subject to erroneous readings caused by electromagnetic or radio frequency interference. Moreover, these and other types of known electronic sensors are of the "active" type and thus require a constant source of electrical power. Active systems are subject to failure when there is an interruption of electrical supply current or malfunctions of the associated electrical components.
Finally, most known liquid level sensors are designed to be installed in the tank at the time the tank is assembled, and thus are not suitable for retrofit to existing tank or reservoir systems.
Accordingly, there is a clear need in the art for a liquid level sensing system which overcomes each of the deficiencies of the prior art discussed above.